Polycarbonate preparation by a phase boundary process has been described by Schnell, “Chemistry and Physics of Polycarbonates”, Polymer Reviews, Volume 9, Interscience Publishers, New York, London, Sydney 1964, pages 33-70; D. C. Prevorsek, B. T. Debona and Y. Kesten, Corporate Research Center, Allied Chemical Corporation, Morristown, N.J. 07960: “Synthesis of Poly(ester Carbonate) Copolymers” in Journal of Polymer Science, Polymer Chemistry Edition, Vol. 18, (1980)”; pages 75-90, D. Freitag, U. Grigo, P. R. Müller, N. Nouvertne, BAYER A G, “Polycarbonates” in Encyclopedia of Polymer Science and Engineering, Volume 11, Second Edition, 1988, pages 651-692, and finally by Dres. U. Grigo, K. Kircher and P. R- Müller, “Polycarbonate [Polycarbonates]” in Becker/Braun, Kunststoff-Handbuch [Plastics Handbook], volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester [Polycarbonates, Polyacetals, Polyesters, Cellulose Esters], Carl Hanser Verlag Munich, Vienna 1992, pages 118-145. A phase boundary process for the preparation of polycarbonate is also described in European Patent Publication No. EP0517044A and European Patent Publication No. EP0520272A, the entire contents of each of which are incorporated herein by reference.
For the preparation of polycarbonate by the phase boundary process, the phosgenation of a disodium salt of a bisphenol or a mixture of different bisphenols, initially introduced into aqueous alkaline solution or suspension, is effected in the presence of an inert organic solvent or solvent mixture which forms a second organic phase in addition to the aqueous phase. The resulting oligocarbonates mainly present in the organic phase are condensed with the aid of suitable catalysts to give high molecular weight polycarbonates dissolved in the organic phase, it being possible to control the molecular weight by suitable chain terminators (monofunctional phenols). The organic phase is finally separated off and the polycarbonate is isolated therefrom by various working-up steps.
Continuous processes for the preparation of condensates using phosgene—for example the preparation of aromatic polycarbonates or polyester carbonates or their oligomers—by the two-phase boundary process have as a rule the disadvantage that, for accelerating the reaction and/or improving the phase separation, more phosgene has to be used than is required for the product balance. The excess phosgene is then degraded in the synthesis in the form of byproducts—for example additional sodium-chloride or alkalicarbonate compounds. Typically, the phosgene excess of about 20 mol % based on the added diphenolate, is used for the continuous two-phase boundary process for the preparation of aromatic polycarbonates according to the prior art (cf. D. Freitag, U. Grigo, P. R. Müller, N. Nouvertne, BAYER A G, “Polycarbonates” in Encyclopedia of Polymer Science and Engineering, Volume 1, 1, Second Edition, 1998, pages 651-692).
In a reduction of the excess phosgene, undesired secondary effects occur, such as poor separation of the dispersion after the last reaction step, and hence increased water content in the organic solution or increased residual monomer or chain terminator contents in the waste water. Various methods for reducing the excess phosgene are discussed in the literature.
DE-A 2 725 967 discloses that it can be advantageous for the phosgene yield of a process first to combine aqueous and organic phase, which contains phosgene, in a tube and then to introduce said phases into a reactor of the tank type. The residence time in this tube should be between 0.5 and 15 seconds. The phosgene excess of the reaction is more than 10 mol %. This reduced phosgene excess requires a disadvantageous phase ratio of organic phase (i.e. oil) to water in order to achieve an effective separation of the two phases after the end of the reaction. Another disadvantage is the relatively long residence time for the phosgenation.
EP-A-304 691 describes a continuous phase boundary process for the preparation of polycarbonates. Here, an aqueous phase comprising diphenols and just the amount of alkalihydroxide required is combined with a phosgene-containing organic phase in a tube with the use of a static mixer. The phosgene excess of 20 to 100 mol % is very high and the residence time in the reaction tube for the first reaction step is 10 to 75 s. With this process, only prepolymers having a molecular weight of 4000 to 12 000 g/mol can be prepared.
EP 0 517 044 A2 describes the reduction of the phosgene excess by means of an annular hole nozzle and a flow tube, just sufficient sodium hydroxide solution being used so that BPA still remains dissolved. This procedure requires complicated regulation with measurement of the chlorocarbonic acid ester groups by means of an ATR crystal and regulated subsequent metering of sodium hydroxide solution to prevent overacidification of the reaction solution. Furthermore, this reaction requires a phase ratio of oil to water that forms a water-in-oil dispersion (oil/water phase ratio greater than 1). The residence time in the flow tube is at least several seconds.
EP 0 520 272 B1 discloses that a small phosgene excess can be achieved by splitting the stream of the BPA solution. Here, part of the BPA solution is mixed with the phosgene solution via a nozzle so that, in this step, a phosgene excess of at least 20 mol % is used. The mixture then reacts further in a flow tube with a minimum residence time of 3 s. Here too, it is required that the dispersion be a water-in-oil dispersion. The disadvantage of the process consists, inter alia, also in the greater effort for metering a second BPA stream.
A reduction of the phosgene excess in preparation of polycarbonate employing the phase boundary processes—where this was possible at all—was therefore achievable to date only with phase ratios disadvantageous for the subsequent separation of the two phases, complicated metering or regulation technology or long residence times of the reactants in the reaction apparatuses.
A reduction of the phosgene excess in the preparation of polycarbonate by the phase boundary processes is, however, important in that such a process will ensure a substantially smaller phosgene loss through subsequent destruction of the excess and moreover small amounts of byproducts would result from this destruction.